1. Field of the Invention
The present invention relates generally to the transport of low molecular weight hydrocarbons under high pressure by ship or barge.
2. Background Art
A number of concepts have been advanced in recent years to produce and transport lighter hydrocarbons (c1 through c7) offshore in a form that is relatively dense such that it becomes suitable for transportation by ship. This may be achieved by cooling the gas and compressing the gas to a modestly high pressure of 1 to 2 MPa (U.S. Pat. No. 5,199,266) or it may be achieved by compressing the gas to a high pressure in special containers (PCT WO 98/14362). The latter system also benefits from using a low temperature during the transport.
An object of the present invention is to achieve a high density of transported natural gas by compressing it to high pressures typically above 5 MPa to transport the gas in a modified composition that permits a very low compressibility factor at near ambient temperature either above or below. This reduces greatly the size of the cooling systems that are required with the present technologies. In some cases cooling of the compressed gas may be achieved in a simple heat exchanger cooled by air or water.
The invention is based on the observation that an ideal gas that is transported under pressure requires a constant ratio between the weight of the containing pressure vessel and the gas regardless of pressure when the strength of the pressure vessel materials and the gas temperature remain constant. However, hydrocarbon gas mixtures are not ideal gases and may deviate from the ideal by a so-called compressibility factor z that in certain circumstances may attain a value of z=0.33 or lower. Thus in the example of z=0.33 the ratio of gas weight to container weight is 3 times that of the corresponding ideal gas. When mixing another gas into the gas being transported the number of molecules to be transported increases, however the value of compressibility factor z may decrease. The increase in total number of molecules by adding the mixing gas reduces the quantity of transport gas that can be carried and the reduction in z increases the quantity of transport gas (and of mixing gas) that can be carried. Later in this specification is shown an example in which that the quantity of transport gas that can be carried at a given temperature and pressure increases more than 50% compared to case in which no mixing gas is mixed into the transport gas.
The condition of transport, i.e. pressure and temperature may be such that the mixture is carried at a temperature below the critical temperature, but above the critical pressure in which case the mixture is transported in the so called dense phase.
The transport of the gas takes place in self propelled ships or non-self propelled barges fitted with a cargo containment system capable of storing the cargo at high pressures, typically above 5 MPa and usually not above 25 MPa. Offshore such vessels are normally loaded at a single point mooring or a multi-buoy mooring connected by subsea pipeline to a process platform. Similar systems are often used when the vessel is loaded from or discharges to facilities on land. The vessels may also be loaded and/or discharged at ordinary fixed berths.
The invention is also applicable to transport of natural gas under high pressure in railroad cars and trucks.
When the mixture of gasses has a molecular weight below about 20 it may not be possible to achieve dense phase at ambient temperatures in the range of 0 to 40 deg C. The transport vessel in consequence may carry a store of higher molecular weight gases (c2 through c7) that when mixed with the incoming cargo results in a molecular weight of the mixture of at least 22 and possibly as high as 28 or higher. The store of higher molecular weight cargo may be gained from gases that condense during discharge of the vessel at its destination due to the adiabatic cooling of the cargo during discharge. These liquids may be retained aboard and transported back to the origin. If insufficient quantities of heavy gases are available at the origin they may be loaded at the destination. If required, the composition of the heavy gases transported back to the origin may be changed through partial discharge or partial receipt of additional hydrocarbons or a combination thereof at the destination point.
The natural gas to be transported is sometimes available at pressures as low as 2 MPa or even lower. Compression of the gas is therefore required prior to being loaded aboard the transport ship. The heat of compression causes significant increases in temperature of the gas. In order to increase the density of the transported gas it may be cooled. The required cooling of the compressed gas may partly or fully take place through exchange of heat through the wall of a submarine pipeline between the compressor and the loading facility. Through this process the gas may reach a temperature that is slightly above the seawater temperature at the seabed before reaching the ship. In the event that no submarine pipes are used the compressed gases may be cooled in an air-cooled heat exchanger and subsequent adiabatic expansion into the storage vessel may result in a final temperature near ambient when the transport pressure is reached. Low temperatures in the storage vessel result in a higher density of the gas being transported. However, it may be advantageous for reasons of safety to maintain a transport temperature slightly above ambient in order to prevent actuation of safety relief valves in accident conditions where the lowered temperature cannot be maintained.
This invention teaches the mixing of a mixing gas into the gas to be transported (transport gas) when it is loaded onto the transport vehicle. The mixing gas is comprised of hydrocarbons and typically has a higher molecular weight than the transport gas.
All heavier hydrocarbons in the mixing gas can be recovered at the destination through known technologies and re-loaded aboard the transport vessel for transport back to the origin. Thus even pure methane can be transported at higher transport densities at near ambient temperatures by being mixed with heavier hydrocarbons at the origin that are recovered from the mixture at the destination.